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Radius of electron trajectory

  1. Nov 18, 2012 #1
    1. The problem statement, all variables and given/known data

    Electrons (mass m, charge –e) are accelerated from rest through a potential difference V and are then deflected by a magnetic field B that is perpendicular to their velocity. Find the radius of the resulting electron trajectory.

    2. Relevant equations
    [tex]F=\dfrac{mv^2}{r}[/tex] [tex]F=qvB[/tex]


    3. The attempt at a solution

    I don't know where to start with this. The electron moves through the potential difference, and presumably gains kinetic energy, but I don't know how to express this in the equations above to get an answer.

    Help appreciated.
     
  2. jcsd
  3. Nov 18, 2012 #2
    I am assuming that the e is accelerated through a gap across which there is a potential difference V. What is the width of the gap?
     
  4. Nov 18, 2012 #3
    That's all the info I'm given; I've literally copied and pasted the question.

    I'm not being asked to give a numerical answer though. It's a multiple choice question, and this is apparantly the answer:

    [tex]\frac{\sqrt{2mV/e}}{B}[/tex]
     
  5. Nov 18, 2012 #4
    The p.d. gives an energy of Ve to the electron.

    From this one can find the resulting velocity of the electron.
     
  6. Nov 18, 2012 #5
    Aha! Got it. Thanks for the help :)
     
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